Polarizers included in polarizing plates are optical devices for polarizing natural light or arbitrarily polarized light in a particular direction, and are widely used in display devices such as liquid crystal displays (LCDs) or organic light emitting diode (OLED) displays. Currently, polyvinyl alcohol-based polarizing films, containing an iodine-based compound or a dichroic dye and having a molecular structure in which molecular chains are oriented in a particular direction, are generally used as polarizers of display devices.
Polyvinyl alcohol-based polarizing films are manufactured by dyeing a polyvinyl alcohol-based film with iodine or a dichroic dye, stretching the polyvinyl alcohol-based film in a particular direction, and performing a cross-linking process thereon. In this case, the stretching process may be a wet stretching process using a solution such as an aqueous boric acid solution or an aqueous iodine solution or may be a dry stretching process performed in the atmosphere, wherein the stretching ratio of the stretching process may be generally 5 times or greater.
In such a manufacturing process, the thickness of a polyvinyl alcohol-based film may be 60 μm or greater before stretching so as to prevent breakage during stretching. If the thickness of the polyvinyl alcohol-based film is 60 μm or less before stretching, the degree of swelling of the polyvinyl alcohol-based film may increase, and the polyvinyl alcohol-based film may easily be broken during stretching because the modulus of electricity of the polyvinyl alcohol-based film per unit area is high due to a thin thickness.
Recently, polarizing plates have been required to have reduced thicknesses to allow for the use thereof with thinner display devices. However, if polyvinyl alcohol-based films having a thickness greater than 60 μm are used as described above, there is a limit to reducing the thickness of polarizers. Thus, much research has recently been conducted into developing thinner polarizers.
Korean Patent Application Laid-open Publication No. 2010-0071998 discloses a method of manufacturing a thin polarizing plate by using a laminate, the laminate being formed by coating a base layer with a hydrophilic polymer layer or coextruding a base layer forming material and a hydrophilic polymer layer forming material. However, if such a coating method or a coextrusion method is used, it may be difficult to separate a polyvinyl alcohol layer and a base layer from each other after stretching, and since a large amount of peel force is used for separating the layers, the polyvinyl alcohol layer may easily be damaged or deformed during separation. As a result, optical characteristics of a polyvinyl alcohol-based film such as a degree of polarization may deteriorate.
Furthermore, if a coating method or a co-extrusion method is used, since a polyvinyl alcohol resin is melted and extruded or is made into a coating solution and applied, the physical properties of a polyvinyl alcohol film may be easily varied according to extrusion conditions, coating conditions, or film-forming conditions. Therefore, the physical properties of final polyvinyl alcohol products may deteriorate, and it may be difficult to obtain final polyvinyl alcohol products having uniform physical properties.